Apparatus and process for regulating current flow through material



Aug. 15, 1961 M. KNIAZUK ET AL APPARATUS AND PROCESS FOR REGULATINGCURRENT FLOW THROUGH MATERIAL Filed March 31, 1959 FRED R PKED/GE? BYzzn I 8 M C. m fi lz, ATTORNEYS United States Patent Ofiicc PatentedAug. 15, 1961 2,996,595 APPARATUS AND PROCESS FOR REGULATING CURRENTFLOW THROUGH MATERIAL Michael Kniazuk, Mountainside, and Fred R.Prediger,

Westfield, N.J., assignors to Merck & Co., Inc., Rahway, N.J., acorporation of New Jersey Filed Mar. 31, 1959, Ser. No. 803,256 '5Claims. (Cl. 219-20) This invention relates to a process and apparatusfor regulating current flow through material characterized by a negativeresistance-temperature coefficient, and in particular, relates to aprocess and an improved power supply for growing a semi-conductor ingot,such as silicon, by a gas phase process wherein an electric current flowthrough a specimen of the semiconductor heats same for carrying out thegrowing process.

In accordance with the present state of the art, an ingot of silicon maybe grown out of a gas phase process by supporting a filament-likespecimen of the silicon in a gas reaction chamber containing a silicongas. The gas is decomposed by heat which deposits additional silicon onthe hot specimen to cause it to grow in diameter. In accordance with theimprovements claimed herein, the heat required for growing thesemiconductor is generated by a controlled electric current flowingthrough the silicon filament. This technique provides a convenient andeffective method of heating and controlling the temperature of the hotsilicon. Silicon has a negative resistancetemperature coeificient. Theresistivity of silicon is relatively very high at room temperature;however, its resistance undergoes a large change as the temperature ofthe hot silicon increases. This phenomenon permits a large temperaturegenerating current to flow through the hot silicon filament for thepurpose of growing same.

It is a principal object of this invention to provide an improvedprocess and apparatus for regulating current flow through a specimen ofmaterial characterized by a negative resistance-temperature coefficient,and in particular, for heating and controlling the temperature of thematerial by such current flow.

It is a further object of the invention to provide an improved processand apparatus for growing semiconductor material, for example silicon,in a gas phase process wherein the temperature required for carrying outsuch process is generated and controlled by regulating the current flowthrough a specimen of such semiconductor.

In accordance with the foregoing objects, it is a further object of theinvention to provide a power supply for regulating current flow througha specimen of material such as silicon or other like materialcharacterized by a negative resistance-temperature coefficient. r Thepower supply circuit includes a step-down transformer wherein thespecimen is coupled alternatively to a high voltage side of thetransformer and then to its low voltage side whereby during the firstphase of operation, a relatively low but increasing current is caused toflow through the specimen. During a later phase of operation, arelatively high current is caused to flow through the specimen. Thepower supply circuit also includes an inductive impedance for varyingthe transformer terminal voltages to :egulate current flow through thespecimen during either phase of operation.

In accordance with the preceding object, it is a further object of theinvention to employ a saturable reactor raving a variable D.-C. bias asthe aforesaid impedance neans, whereby variation of the D.-C. biaseffects reguation of current flow through the specimen.

As noted hereinbefore, the cold silicon has a rela- :ively very highresistivity at room temperature. Ac- :ordingly, it is a further objectof the invention to heat the specimen of silicon filament by externalmeans to facilitate initial current flow therethrough at the start ofthe gas phase process. External heating may be discontinued aftercurrent flow through the specimen increases sufficiently whereby thefilament is able to heat itself.

Further objects and advantages will become apparent from the followingdescription of the invention taken in conjunction with the figures, inwhich:

FIG. 1 depicts schematically the power supply in accordance with thepractice of the instant invention; and

FIG. 2 illustrates the B-H characteristics of the saturable reactor,which curve is used herein to describe the operation of the reactor.

Reference is now made to the figures wherein FIG. 1 illustrates arod-like silicon filament 10 supported by conventional means in avacuum-sealed reaction chamber 12. In accordance with the present stateof the art, a gas mixture 13 including a carrier gas and a silicon gas,such as silicon tetrahalide is introduced into chamber 12 via an inlet14. The silicon gas mixture 13 when suitably heated will decompose todeposit additional silicon on the hot filament 10. This action causesfilament 10 to grow in diameter. The exhaust gases in chamber 12 areremoved through an outlet :15.

The present invention is primarily concerned with the means for heatingthe silicon mixture 13 and filament 10 by regulating current flowthrough the latter. Current flow is regulated by the power supplydepicted in FIG. 1, which power supply is energized by an A.-C. source16, for example 440 volts. The power supply includes a stepdowntransformer 17 and a saturable reactor 18, which reactor is connected inseries with the high voltage side of transformer 17. Reactor 18 isessentially a variable inductive impedance, the inductance of which maybe varied considerably by a variable direct current bias in thesecondary winding circuit of reactor 18. Specifically, transformer 17and reactor 18 each have respective primary and secondary windings -19,20 and 21, 22. The series connected primary windings 19, 21 areconnected across source 16. The D.-C. bias for reactor 18 involves aregulated variable D.-C. source depicted at 23 and connected to reactorwinding 22.

The path for current flow through filament 10 includes a line 25connecting filament 10 to one side of source 16. For the purpose ofidentification, this side of source 16 often will be referred to as theupper side, whereas the other side of source 16 will be referred to asthe lower side. The current path continues through filament 10 and line27, which line is connected to a single pole double throw switch 28having terminals 29, 30. Terminal 29 connects to the lower side ofsource 16 through reactor winding 21. It will be understood that theconductive connections of lines 25 and 27 to filament 10 may beaccomplished by known conventional means.

When switch 28 closes to contact terminal 29, filament 10 is essentiallyconnected across a relatively high voltage and low current source asconstituted by transformer primary 19. For this switching condition, thesecondary winding circuit of transformer 17 is open, and primary winding19 draws a negligible magnetizing current for the no-load condition. Theterminal voltage applied to primary 19 will be maximum for a minimumvoltage drop across reactor 18 and minimum for a maximum voltage dropacross reactor 18. When switch 28 closes contact with terminal 30,filament 10 is essentially across a low voltage and high current sourceas constituted by the terminal voltage of transformer secondary 20. Thehigh current path is depicted in FIG. 1 by heavy bus lines. For thereasons explained hereinafter, it will be seen that filament 10 isconnected to the high voltage 3 side of transformer 17 during an earlyphase of current flow, and is connected to the low voltage and highcurrent side of transformer 17 during a later phase of current flow.Meanwhile, reactor 18 will serve to vary the primary and secondarytransformer terminal voltages to control continuously current flow infilament for either position of switch 28.

Reference is now made to FIG. 2, which curve illustrates the 8-Hcharacteristics of the magnetic core of saturable reactor 18, where Bdenotes fiux density and H denotes the magnetizing force. The reactorinductance may be defined as follows:

where is core flux, N is the number of turns, and I denotes cur-rent.Since 1: and I are directly proportional to B and H, respectivelyAlfi/AI is essentially the slope of the B-H curve. Consequently, reactorinductance will be a maximum for operating conditions in the region ofthe origin of the curve coordinates wherein average core flux is zero.On the other hand, reactor inductance decreases for reactor operation inthe region where the curve flattens out by reason of an increase inaverage core flux. The D.-C. bias in the secondary of reactor 18determines average core flux, which in turn determines the reactorinductance L, whereby a decrease in D.-C. bias results in an increase ofreactorinductance, and conversely an increase of D.-C. bias results in adecrease of inductance.

At the start of the gas phase process of growing the silicon, filament10 is at room temperature wherein the silicon exhibits a very highresistance. Consequently, it is desirable to apply the highest availablevoltage across filament 10 for the initial phase of current flow therethrough. This is accomplished by throwing switch 28 to close contact 29and by biasing reactor 18 with a high DC. bias. The latter actionminimizes the voltage drop across reactor 18. Nevertheless, theextremely high resistance of cold silicon will prevent a sufficient rateof current flow build-up through filament 10 to permit the silicon toheat itself. Consequently, external heating means 31 such as a bank ofheating elements are disposed in close spaced relationship about chamber12. Heaters 31 may be connected across source 16 by a switch 32 andserve to heat filament 10 by radiation.

Consequently, it will be understood that at the start of the foregoingprocess, the voltage applied to filament 10 is the highest available,and that the initial phase of current flow through the silicon filamentis facilitated by heating same by external means. The resistance of theheated filament drops which results in increasing current flowtherethrough. This action leads to a higher filament temperature. Whenthe current is sufficiently large-to self-heat filament 10, heater 31may be disconnected from the circuit by opening switch 32. Thecontinuation of current flow through filament 10 is characterized by anincrease of its temperature accompanied by further increases in current.To prevent a runaway condition which will melt the silicon, the processmay be controlled by decreasing the DC. reactor bias in order toincrease reactor inductance. This adjustment will reduce the terminalvoltage applied to transformer 17 and in turn the voltage acrossfilament 10 to regulate the current flow therethrough. In actualpractice, the D.-C. bias may be varied manually to increase the reactorinductance by suitable increments at predetermined time intervals. Itwould be keeping within the spirit of this invention to obtain such biasadjustments automatically by a control regulated by a thermostatresponsive to the temperature of chamber 12.

When the voltage across filament 10 drops to a predetermined amount, forexample 80 volts, switch 28 may be thrown to close terminal 30 in orderto cause a large heat generating current to flow through the siliconfilament. This switch action couples filament 10 to the low voltage sideof transformer 17. During the phase of large current flow, reactor 18 isagain employed to control the voltage applied across filament 10 andthus the current flow therethrough in order to obtain and, in addition,to maintain a suitable silicon temperature in chamber 12 for growing thesilicon ingot. At the completion of the growing process, the powersupply circuit may be opened, for example by a switch 33, whereby thecompleted ingot is replaced by another silicon specimen and theforegoing process is repeated.

It is intended that all matter contained in the above description orshown in the accompanying drawings shall be interpreted as illustrativeand not in a limiting sense.

We claim:

1. A power supply for regulating the current flow through a conductorwork specimen characterized by a negative resistance-temperaturecoefi'icient comprising, a source of A.-C. voltage, a step-downtransformer having a primary winding and also a normally open circuitsecondary winding, said primary winding being coupled to said-source,variable inductor reactance means in electrical series relationship withsaid primary winding for varying the terminal voltage applied to saidconductor work specimen, switching means for conductively connecting theconductor work specimen alternatively across said primary or secondwinding wherein said conductor work specimen is connected to arelatively high voltage and low current source or a relatively lowvoltage and high current source, respectively, said conductor beingconnected to the high voltage and low current source during an earlyphase of current flow therethrough and; being connected to the lowvoltage and high current, source during a later phase of current flowtherethrough, 5 and means for regulating the inductance of saidinductor; means wherein the voltage applied across said transformerprimary is varied to regulate current flow through said conductor workspecimen for either phase of opera-- tion.

2. A power supply for regulating the heating currentflow through asemiconductor specimen characterized: by a negativeresistance-temperature coeflicient comprising, a source of A.-C.voltage, a step-down transformer having a primary winding and also anopen circuit secondary winding, said primary winding being conductivelycoupled to said source, a saturable reactor in electrical seriesrelationship with said transformer primary winding for varying theterminal voltage applied to said semiconductor specimen, means forconnecting said specimen, of semi-conductor alternatively across saidprimary or secondary winding of said transformer wherein said, specimenis connected to a relatively high voltage and low current source or arelatively low voltage and high current source, respectively, saidspecimen being 6011-; nected to the high voltage source during an earlyphase, of current flow therethrough and being connected to the lowvoltage source during a later phase of current flow. therethrough, andmeans for regulating the inductance of said reactor wherein the voltageacross said specimen is controlled to regulate current flow therethroughfor either phase of operation.

3. Apparatus as defined in claim 2 further including, heat generatingmeans in close spaced relationship with respect to said specimen and forheating said specimen by radiation until the temperature of saidspecimen rises to facilitate a buildup of initial current flowtherethrough.

4. A power supply for regulating the heating current flow throughsilicon or like semiconductor specimen material which is characterizedby a negative resistance-temperature coefficient comprising, a source ofA.-C. voltage a step-down transformer having a primary winding am alsoan open circuit secondary winding, a saturable reac tor having a primarywinding in electrical series relationship with said transformer primarywinding and alsc having a secondary winding for varying the terminalvoltage applied to said specimen, said source being conductively coupledacross the aforesaid series combination of primary windings, means forconnecting said specimen of semiconductor alternatively to said primaryor secondary winding of said transformer wherein said specimen isconnected to a relatively high voltage and low current source or arelatively low voltage and high current source, respectively, saidspecimen being connected to the high voltage source during an earlyphase of current flow therethrough and being connected to the lowvoltage source during a later phase of current flow therethrough, andvariable direct current bias means coupled to said reactor secondarywinding for regulating the inductance of said reactor wherein thevoltage applied to said transformer primary is varied to regulatecurrent fiow through said specimen for either phase of operation.

5. In apparatus for applying an alternating current from a substantiallyconstant voltage source to a relatively cold negative-resistance workspecimen to heat same and for regulating the current flow through saidwork specimen as its temperature rises wherein said apparatus includes atransformer in which the secondary winding has a smaller number of turnsthan the primary winding, and said apparatus also including anadjustable reactor, the improvement in said apparatus comprising, meansconnecting said adjustable reactor and the primary winding of saidtransformer in series across the source of alternating current, meansconnecting one terminal of said work specimen to one terminal of saidsource of alternating current, switch means connecting the otherterminal of said work specimen to either the series connection betweenthe adjustable reactor and transformer primary winding or one terminalof the secondary winding of said transformer, and means connecting theother terminal of said transformer secondary winding to said oneterminal of said work specimen, said work specimen is energized fromsaid alternating current source through said adjustable reactor whensaid switch means connects said other terminal of said work specimen tothe series connection between the adjustable reactor and said primarytransformer winding, whereby said adjustable reactor serves to controlthe amount of current flow through said work specimen, said workspecimen is energized from said alternating current source through saidtransformer secondary winding when said switch means connects said otherterminal of said work specimen to said one terminal of said secondarytransformer winding, and whereby said adjust-able reactor in series withthe primary winding of said transformer also serves to control thecurrent flow through said work specimen.

- References Cited in the file of this patent UNITED STATES PATENTS1,913,580 Altshuler et al June 13, 1933 2,314,956 Slayter et a1. Mar.30, 1943 2,338,518 Koch Jan. 4, 1944 2,680,771 Kistler June 8, 19542,769,076 Bogdan Oct. 30, 1956

